Family of compounds crosslinkable by photon irradiation

ABSTRACT

A family of negative resins, capable of undergoing crosslinking under the effect of more or less energetic photons (gamma radiation, X-rays, ultraviolet or visible light), applicable to the protection of objects against atmospheric agents and to the production of masks of the type used in the production of integrated circuits. The typical compound according to the invention contains at least one substance of which the chemical formula comprises a thiirane ring: ##STR1## such as 2,3-epithiopropyl methacrylate copolymerized with a vinyl monomer, such as methyl methacrylate. Crosslinking is facilitated by photoinitiators, such as aryl diazonium and aryl iodonium salts liberating Lewis acids. By selecting the photoinitiator, it is possible to act on the spectral region where irradiation is effective.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of our earlier application Ser. No.163,479 filed June 27, 1980, now abandoned, which in turn is a divisionof our earlier application Ser. No. 67,905 filed Aug. 20, 1979, now U.S.Pat. No. 4,285,788, which in turn is a division of our application Ser.No. 882,169 filed Feb. 28, 1978, now U.S. Pat. No. 4,259,162.

BACKGROUND OF THE INVENTION

This invention relates to a family of compounds which can be crosslinkedby photon irradiation and to a process for using these compounds. Thephotons used for irradiation are photons issuing from either visible orinvisible radiation, particularly ultraviolet radiation, X-radiation orgamma radiation.

The phenomenon of crosslinking of so-called negative resins is wellknown. Resins such as these harden under the effect of photonirradiation by the establishment of "bridging" bonds between the variouspolymer chains which they contain. These bonds result in the formationof a three-dimensional network. From the point of view of the intrinsicproperties of the resins, this is reflected in an increase in theirmolecular weight which is responsible for their insolubility indeveloper solvents.

Commercially available crosslinkable resins are formed:

either by a polymer containing both a crosslinkable group and aphotosensitive group:

or by a "system" containing both a polymer having a crosslinkable entityand a photoinitiator or photosensitiser compound.

The sensitivity of two known crosslinkable compositions is given in thefollowing by way of order of magnitude:

4 mJ/cm² (in photon irradiation at a wavelength of 404.7 nm) for theresin KPR (a Kodak product);

1.3 mJ/cm² (in photon irradiation at wavelengths of 351.1 and 363.8 nm)for the epoxy resin commercially available under the name "PSE-2"; asensitivity of this order requires an exposure time of around 10 secondsunder normal working conditions in a factory.

By virtue of the present invention, it is possible to obtain excellentsensitivity levels, i.e. in favourable cases of the order of one tenthof a millijoule per square centimeter.

In addition, the compounds according to the invention have otheradvantages which will be discussed hereinafter.

According to the invention, there is provided a family of compoundscrosslinkable by photon irradiation comprising at least one copolymer ofwhich the chemical formula comprises a thiirane ring: ##STR2## whereinsaid polymer contains at least one monomer formed by 2,3-epithiopropylmethacrylate corresponding to the developed formula: ##STR3## wherein Ris selected from the group consisting of an alkyl group of 1 to 4 carbonatoms and hydrogen.

According to one aspect of the invention, the above monomer iscopolymerized with a vinyl monomer corresponding to the formula:##STR4## in which R' is either a radical H or an alkyl group C_(n)H_(2n) +1 where n is an integer from 1 to 10, R" being an alkyl groupcontaining from 1 to 5 carbon atoms.

According to another aspect of the invention, said family additionallycomprises at least one of the following two salts:

an aryl diazonium salt corresponding to the general formula: ##STR5##and an aryl iodonium salt corresponding to the formula: ##STR6## wheren=0 or 1.

In these formulae:

the substituent Δ denotes a radical or a plurality of radicals such as:

--OH; --NH₂ ; --CHO; --NO₂ ; --OCH₃ ; ##STR7##

the letters a and b denote integers from 1 to 5;

the element M is a metal such as Fe, Sn, Sb or Bi or another elementsuch as B, P, As;

X denotes a halogen atom, such as F or Cl;

T₁ and T₂ denote aromatic radicals (which may be identical) containingfrom 4 to 20 carbon atoms (phenyl, thienyl, furanyl, pyrazolyl);

Y denotes a radical of the following group: ##STR8##

R₃ --N (with R₃ =H or an alkyl or acyl radical or a C--C bond); ##STR9##(where R₁ and R₂ represent identical or different radicals such as H ora C₁₋₄ -alkyl radical or a C₂₋₄ -alkenyl radical).

The copolymer of 2,3-epithiopropyl methacrylate and the vinyl monomermay be obtained as follows:

Glycidyl methacrylate corresponding to the formula: ##STR10## is reactedwith thiourea corresponding to the formula: ##STR11## at ambienttemperature in a mixture of water and ethanol. The reaction gives2,3-epithiopropyl methacrylate corresponding to the formula: ##STR12##

This compound is extracted with ether. The solution is dried with sodiumsulphate. The ether is removed in vacuo and the remaining product isdistilled in vacuo.

The epithiopropyl methacrylate thus obtained is mixed with a quantity ofvinyl monomer in a proportion of from 20 to 60% by weight thereof in asolvent, such as benzene. Azobis-isobutyronitrile, which acts ascopolymerisation catalyst, is then added to the resulting mixture. Themixture is heated under nitrogen at 80° C. over a period ranging fromone to several hours. After cooling, the copolymer is obtained byprecipitation with methanol, followed by drying in vacuo.

For example, the vinyl monomer may be butyl methacrylate orethylacrylate.

The invention will be better understood from the following descriptionand examples which in particular define a process for using the resin.

In the formula of the diazonium salt, the ion:

(M X_(b+a))^(a-)

is a complex halogenated anion (X=F or Cl) with a metallic ornon-metallic element M (such as defined above) of which the charge is a.The number of negative charges also represents the number of halogenatedions complexed with the Lewis acid M X_(b).

A similar situation with a single negative charge on the halogenatedanion prevails in the case of the diaryl iodonium salt.

By photolysing, these salts produce the Lewis acids M X_(b) whichchemically initiate the crosslinking process by a cationic mechanism.This mechanism consists in the formation of a covalent bond between thesulphur atom of the thiirane cycles, which gives an electron pair, andthe element M which accepts this electron pair.

The formation of a complex having an ionic tendency, which results fromthe more intensive polarisation of one of the C--S bonds of the thiiranerings, enables the opening of other thiirane rings to be initiated,giving rise to a cationic crosslinking reaction: ##STR13##

The phase (3) is the initiation phase; the phase (4) is the propagationphase.

Since the thiirane ring is not (or only slightly) affected by agentsforming free radicals, but is highly sensitive to ion attacks, theradiation dose required for crosslinking the "thiirane" resins in thepresence of Lewis acids is lower then when they are irradiated on theirown.

The photocrosslinking of the compounds according to the invention is areaction of the initiated type because, once the degradation of thephotoinitiators (aryl diazonium or aryl iodonium salts) has beeninitiated, i.e. after formation of the Lewis acids, irradiation may bestopped and the crosslinking reaction may continue in the absence ofirradiation like any polymerisation reaction initiated by Lewis acids.

Although the resins according to the invention used on their use canonly be sensitised in a narrow spectral region, the addition of theabove-mentioned photoinitiators not only enables sensitivity to beincreased, it also enables the spectral sensitivity range to be widenedby extending it in some cases into the range of visible light.

Thus, by acting on the substituent of the aryl group in the case of thearyl diazonium salts, the absorption range is displaced towards thevisible. For example, with a substituent such as a halogen atom or analkyl radical, a slight displacement is obtained towards the majorwavelengths (up to 273 nm for the substituent Cl). This displacementbecomes very considerable when groups containing non-binding electrons,such as

--OH; --NH₂ ; --CHO; --NO₂ ; --OCH₃ ##STR14## are substituted.

In order to obtain sensitisation in the region of visible light, it isnecessary for example to add to the above-mentioned products so-called"photo-optical" products which serve to sensitise the photoinitiators(for example the aryl diazonium and aryl iodonium salts). In general,sensitisation is obtained by the transfer of energy from the "triplet"state of the sensitiser (donor) to the "triplet" state of the initiator(acceptor), photodegradation taking place under the effect of this inputof energy.

Depending upon the required spectral region of sensitisation, the"photo-optical" products may belong to various families of dyes,provided that these dyes do not destroy the Lewis acids formed duringthe initiation phase and that their energy levels are adapted to thoseof the photoinitiators used so as to provide for an effective transferof energy.

The dyes in question include, for example, coumarins, xanthenes,acridines, thiopyronines, safranines, thiazines, oxazines, cyanines(carbocyanines, oxacyanines, thiacyanines), coloured polycyclic aromatichydrocarbons and, finally, compounds containing para-substitutedaminostyryl groups.

As mentioned above, the compounds according to the invention may becrosslinked by ionising radiation, such as X-radiation or gammaradiation. Their sensitivities to radiation of this type areapproximately ten times higher than those of the corresponding epoxyresins, for example 80 J/cm³ as opposed to 740 J/cm³ for an irradiationwavelength of 8.34 Angstroms.

In the case of irradiation by soft X-rays (spectral region where thephoto-electrical absorption is preponderant over the Compton diffusion),for example for photons with energy levels of from about 0.1 to 10 KeV,it is possible to increase the sensitivity of the compounds according tothe invention to excess in relation to that of epoxy resins byincreasing their coefficient of absorption to the X-rays by optimisationof the irradiation wavelength, as explained hereinafter.

It is known that the sensitivity of a resin to radiation depends uponthe energy of the incident photons which it absorbs. Its so-called"X-photoelectric" absorption coefficient is a weighted sum of theabsorption coefficients of the various atomic species by which it isformed. These coefficients increase with the incident wavelength(proportionally to the third power) and with the atomic number of theabsorbent (proportionally to the fourth power) between thediscontinuities which characterise the photoelectric absorption. Thesediscontinuities which correspond to the ionisation potentials of thevarious energy levels are characteristic of each type of atom.

In order to increase the sensitivity of the compounds according to theinvention to soft X-rays, the wavelength of the X-rays is selected forexample in the spectral region situated beyond a discontinuity of oxygenand within one of the following discontinuities of sulphur.

This is the case between the wavelengths of 76.05 Angstroms(discontinuity L_(II) of sulphur) and 23.30 Angstroms (discontinuity Kof oxygen) where the absorption coefficient of sulphur is respectivelyfrom 7 to 20 times greater than that of oxygen, although these twocoefficients have substantially the same value for shorter wavelengths,i.e. 23,000 cm⁻¹ at 22.3 Angstroms; the absorption coefficient of oxygenbecoming greater than that of sulphur at longer wavelengths (i.e.respectively 55,700 and 24,900 cm⁻¹ at 109 Angstroms).

The following table gives the absorption coefficients of the oxygen andsulphur atoms for certain wavelengths (or photon energy levels) situatedin the range favourable to an increase in sensitivity of the compoundsaccording to the invention, the wavelengths selected being those of thelines of various atoms emitting in the region in question.

    ______________________________________                                                        Nature of                                                                     the line                                                      λ (Å)                                                                     E(KeV)   emitted  .sup.μ O(cm.sup.-1)                                                                .sup.μ S(cm.sup.-1)                                                               .sup.μ S/.sup.μ O               ______________________________________                                        23.6   0.525    O-K.sub.α                                                                        1368    26931  19.7                                  27.4   0.452    Ti-L.sub.α1.2                                                                    2002    37943  18.95                                 31.4   0.395    Ti-L.sub.1                                                                             2825    50798  18.0                                  31.6   0.392    N-K.sub.α                                                                        2881    51605  17.9                                  44.7   0.277    C-K.sub.α                                                                        6890    99236  14.4                                  58.4   0.212    W-N.sub.V,II                                                                           13247   126601 9.6                                   64.4   0.193    MO-M.sub.ζ                                                                        16587   144465 8.7                                   67.6   0.183    B-K.sub.α                                                                        18844   153553 8.15                                  72.2   0.172    Nb-M.sub.ζ                                                                        21842   165145 7.6                                   ______________________________________                                    

In the above-mentioned region, the use of photoinitiators such as thecomplex aryl diazonium and diaryl iodonium salts enables the sensitivityof the compounds containing a thiirane ring to be increased, the X orgamma photons initiating the degradation of these salts into Lewis acidsin the same way.

In addition, in the region of the soft X-rays where the photoelectricabsorption is preponderant, the choice of the photoinitiator,particularly the choice of the element M in the general formula of thecomplex salt, enables the sensitivity of the compound according to theinvention to be increased, as shown hereinafter.

It is known that, in general, the interaction mechanism betweenradiation and sensitive resin uses a transfer of energy from thechemical group or the absorbent atom of the resin towards its sensitivepart. The effectiveness of this transfer is dependent upon the distancebetween these two "entities."

Accordingly, it is possible to use to advantage the narrow distancebetween the absorbing group and the sensitive group in case where theabove mentioned complex salts are used, i.e. the proximity between thesulphur atom of the thiirane ring and the element M of the Lewis acid.Accordingly, this element is selected according to its X-photonabsorption coefficient.

The following table shows the absorption coefficients in cm⁻¹ for fiveelements M in five cases of use characterised by the emission ofdifferent atomic lines.

    ______________________________________                                        λ                                                                              8.34 Å  13.3 Å    23.6 Å                                  Element line K.sub.α1.2 of Al                                                               line L.sub.α1.2 of Cu                                                                 line K.sub.α  of                      ______________________________________                                                                          O                                           Fe      27071       84173         31504                                       Sn      24350       70344         168788                                      Sb      23522       63625         22870                                       Bi      26580       62137         123689                                      As      30942       14359         50453                                       ______________________________________                                                 λ                                                                        44.7 Å 72.2 Å                                              Element    line K.sub.α  of C                                                                 line M.sub.ζ  of Nb                                ______________________________________                                        Fe         104724     221496                                                  Sn         46287      47932                                                   Sb         44167      44181                                                   Bi         92918      32019                                                   As         159867     268336                                                  ______________________________________                                    

A Process for Using the Compounds and Examples of Application

The compounds according to the invention are referred to hereinafter bythe name of "thiirane resin."

Two principal uses of the thiirane resins are described in thefollowing.

The first use is in the production of masks "in situ" during themanufacture of electronic components, these masks being intended forelectronic masking purposes. The technique in question is the techniqueof microlithography used in particular for the production of integratedcircuits.

The second use is in the treatment of various objects with a view tocoating them with a protective layer; this is the case with sheaths ofelectrical cables and articles of cabinet work.

In both cases, the first step is to prepare a solution of thiiraneresines in a solvent, such as 2-butanone.

In the first case, the solution is applied to a substrate or to a wafercontaining one or more integrated circuits in the course of formationusing a centrifugal apparatus. A layer varying in thickness from 200 to500 nanometers for example is obtained, depending upon the rotationalspeed of the substrate or wafer.

Irradiation is then carried out using a source of actinic (visible orultraviolet) light, such as a deuterium lamp, a mercury vapour lamp, axenon lamp, a carbon arc lamp, a tungsten filament lamp or a laser, suchas a laser containing organic molecules (for example an organicscintillator or dye).

In the second case, the procedure adopted may be the same as in thefirst case or may comprise dip coating or spray coating. The layer isgenerally thicker where these last two techniques are used, which ispreferably for obtaining effective protection.

In this latter case, irradiation must be carried out in depth, so thatit is of advantage to use a penetrating radiation, such as soft X-rays.To this end, it is possible to use either a synchrotron or an apparatuscomprising an electron gun and an anticathode selected according to thedesired wavelength: rhodium (line at 4.6 Angstroms), molybdenum (line at5.41 Angstroms), aluminum (line at 8.34 Angstroms), copper (line at13.3. Angstroms), carbon (line at 44.7 Angstroms). Finally, it ispossible to use the gamma rays produced by a conventional cesium- orcobalt-based source.

FIRST EXAMPLE

This example concerns the photocrosslinking of a thiirane resin used onits own with a view to its application in microlithography.

1.5 g of a copolymer of 2,3-epithiopropyl methacrylate and methylmethacrylate (in a ratio of 1:1) are dissolved in 2-butanone to obtain a10% solution.

A substrate of oxidised silicon (500 to 100 Angstroms of silica) is thencoated by centrifuging with this resin.

The substrate thus treated is then subjected to the flux of a highpressure mercury vapour lamp, for example a 125 watt lamp of the HPR 125type manufactured by Philips. The distance between the source and thesample is 12 cm. The flux received by the sample is 4.33 mW/cm² at 365nm. The irradiation time is adjusted by trial and error to obtain 70%crosslinking of the thickness of the layer of resin.

Development is carried out by spraying on a mixture of 2-butanone andethanol in a ratio of 5:1.8 for a period of 30 seconds, followed by thespraying of isopropanol over a substantially identical period.

SECOND EXAMPLE

This example is also concerned with applications in microlithography. Onthis occasion, however, the resin used is mixed with a photoinitiatorwhich, under irradiation, releases Lewis acids which initiate thecrosslinking process.

THe photoinitiator used is a complex aryl diazonium salt, namelyp-diazo-N, N-diethyl aniline hexafluoro antimonate, which has anabsorption in ultraviolet light at 375 nm. This salt is obtained byprecipitation from an aqueous solution of p-diazo-N, N-diethyl anilinefluoborate to which sodium hexafluoro antimonate is added.

An aqueous solution of freshly prepared Na Sb F₆ containing 3.14 g ofthis compound in 2.5 cc of twice-distilled water is added to 3.91 g ofthe above-mentioned fluoborate (i.e 1.21×10⁻² mole) dissolved in 90 ccof twice-distilled water. The precipitate is collected by filtration andthen dried in vacuo. The yield obtained amounts to 55.4%.

In the absence of any actinic radiation, 0.0375 g of the photoinitiatorthus obtained are dissolved in a solution of thiirane resin similar tothat of the first example. The quantity of photoinitiator used iscalculated to obtain a proportion of 5% by weight, based on the weightof the resin.

A substrate of oxidised silicon coated with resin under the sameconditions as in the first example, is irradiated in the same way, butfor a different time, to obtain 70% crosslinking of the thickness of thelayer of resin.

THIRD EXAMPLE

This example is similar to the first example, except that irradiation iscarried out with soft X-rays.

2 g of copolymer of 2,3-epithiopropyl methacrylate and methylmethacrylate (in a ratio of 1:2) are dissolved in 22.3 cc of 2-butanoneto form a solution containing 10% of solute. The deposition of thisresin by centrifuging (at 8000 rpm) onto a substrate of oxidised siliconenables a layer of resin 5200 Angstroms thick to be obtained.

The X irradiation is obtained by the electron bombardment of ananticathode of aluminium with a 300 W gun (intensity 50 milliamperes foran accelerating voltage of 6 kV). The emission observed is that of theK.sub.α1.2 line of aluminum, i.e. a wavelength of 8.34 Angstroms. Theflux X after filtration through a 2 micrometer thick aluminum foil(intended to eliminate the lowest-energy component of the deceleratingradiation) is evaluated at 398.5 microwatts per cm². The irradiationtime is 1 minute 45 seconds corresponding to an absorbed dose of 80 J/ccfor crosslinking 70% of the coated thickness after development by thesame mixture as in the first example and under the same condition.

FOURTH EXAMPLE

This example is similar to the second example except that irradiation iscarried out with soft X-rays.

A solution of thiirane resin (copolymer of 2,3-epithiopropylmethacrylate and methyl methacrylate) is prepared under the sameconditions as in the third example. The photo-initiator of the secondexample is added to the solution in such a quantity that a proportion of5%, based on the weight of the resin, is obtained.

The X-irradiation is carried out under the same conditions as in thethird example. The irradiation time is adjusted by trial and error toobtain the 70% crosslinking of the thickness of the layer of resin.

What we claim is:
 1. A copolymer composition crosslinkable by ionizingradiation into a 3-dimensional network, said composition comprising, asthe cross-linkable monomers:(i) from 40 to 80% by weight of 2,3epithiopropyl alkyl acrylate monomer units of the formula: ##STR15##wherein R is hydrogen or a C₁ to C₄ alkyl; and (ii) from 60 to 20% byweight of vinyl monomer units of the formula: ##STR16## wherein R¹ ishydrogen or an alkyl group of the formula C_(n) H_(2n+1), where n is aninteger from 1 to 10 and R² is a C₁ to C₅ alkyl group, together with(iii) an initiating amount of an aryl iodonium salt of the formula:##STR17## where n is 0 or 1, T₁ and T₂, which are the same or different,are aromatic groups consisting from 4 to 20 carbon atoms, Y is:##STR18## where R₃ is hydrogen, alkyl or acyl, or ##STR19## where R₄ andR₅, which may be the same or different, are hydrogen, a C₁ to C₄ alkylor a C_(2-C) ₄ alkenyl,M is Fe, Sn, Sb, Bi, B, P or As, X is a halogenatom, and B is an integer from 1 to
 5. 2. A process for using thecrosslinkable copolymer composition as defined in claim 1 adapted forforming a mask for producing electronic components, said processcomprising the steps of:(a) dissolving the copolymer composition in apredetermined quantity of solvent; (b) applying the solution formed instep (a) in the form of a thin layer to a predetermined part of thesubstrate of an electronic component, and (c) irradiating predeterminedportions of said substrate thereby crosslinking the irradiated portionsof the film forming a 3-dimensional network.
 3. The process as claimedin claim 2 wherein the solvent in step (a) is 2-butanone, the solutionis applied in step (b) by centrifuging, and ionizing radiation isapplied in step (c).
 4. A process for using the crosslinkable copolymercomposition as defined in claim 1 for forming a mask for producingelectronic components, said process comprising the steps of:(a)dissolving the copolymer composition in a predetermined quantity of asolvent; (b) applying the solution formed in step (a) in the form of athin layer by dip coating or spray coating onto an object to beprotected by the thin copolymer film; and (c) subjecting the thin filmapplied in step (b) to ionizing radiation thereby crosslinking thepolymer composition and forming a 3-dimensional network.
 5. The processof claim 4 wherein soft X-rays or gamma rays are used as the ionizingradiation.
 6. A copolymer composition crosslinkable by ionizingradiation into a 3-dimensional network, said composition comprising, asthe cross-linkable monomers:(i) from 40 to 80% by weight of2,3-epithiopropyl alkyl acrylate monomer units of the formula: ##STR20##wherein R is hydrogen or a C₁ to C₄ alkyl; and (ii) from 60 to 20% byweight of vinyl monomer units of the formula: ##STR21## wherein R¹ ishydrogen or an alkyl group of the formula C_(n) H_(2n+1), where n is aninteger from 1 to 10 and R² is a C₁ to C₅ alkyl group, together with(iii) an initiating amount of an aryl diazonium salt of the formula:##STR22## wherein a and b are integers from 1 to 5 and Δ is selectedfrom the group consisting of --OH, --NH₂, --CHO, --OCH₃ --NO₂, ##STR23##M is Fe, Sn, Sb, Bi, B, P or As, and X is halogen.
 7. A process forusing the crosslinkable copolymer composition as defined in claim 6adapted for forming a mask for producing electronic components, saidprocess comprising the steps of:(a) dissolving the copolymer compositionin a predetermined quantity of solvent; (b) applying the solution formedin step (a) in the form of a thin layer to a predetermined part of thesubstrate of an electronic component, and (c) irradiating predeterminedportions of said substrate thereby crosslinking the irradiated portionsof the film forming a 3-dimensional network.
 8. The process as claimedin claim 7 wherein the solvent in step (a) is 2-butanone, the solutionis applied in step (b) by centrifuging, and ionizing radiation isapplied in step (c).
 9. A process for using the crosslinkable copolymercomposition as defined in claim 6 for forming a mask for producingelectronic components, said process comprising the steps of:(a)dissolving the copolymer composition in a predetermined quantity of asolvent; (b) applying the solution formed in step (a) in the form of athin layer by dip coating or spray coating onto an object to beprotected by the thin copolymer film; and (c) subjecting the thin filmapplied in step (b) to ionizing radiation thereby crosslinking thepolymer composition and forming a 3-dimensional network.
 10. The processof claim 9 wherein soft X-rays or gamma rays are used as the ionizingradiation.